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In the context of an insulation of a wall (old and very irregular) by gypsum plasterboard on metallic frameworks, how is the top rail fixed if there is no ceiling?

A wall without a ceiling seems difficult to isolate.

If it is a sloped ceiling, attic type, it is necessary to put a piece of wood on the ceiling that allows to fix the top rail in the same axis as the bottom but obviously reversed.

The piece of wood will have to catch the slope of the ceiling to present a horizontal face, the one where will be screwed the upper support rail. Take the angle with a false square to make an exact cut of the piece of wood.

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